Re: [GIT PULL] cputime patch for 2.6.30-rc6

Martin Schwidefsky
2009-05-19T08:49:20Z

On Mon, 18 May 2009 17:24:03 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> On Mon, 2009-05-18 at 16:09 +0200, Martin Schwidefsky wrote:
> > Hi Linus,
> >
> > please pull from 'cputime' branch of
> >
> > git://git390.marist.edu/pub/scm/linux-2.6.git cputime
> >
> > to receive the following updates:
> >
> > Michael Abbott (1):
> > Fix idle time field in /proc/uptime
> >
> > fs/proc/uptime.c | 8 +++++++-
> > 1 files changed, 7 insertions(+), 1 deletions(-)
> >
> > diff --git a/fs/proc/uptime.c b/fs/proc/uptime.c
> > index 0c10a0b..c0ac0d7 100644
> > --- a/fs/proc/uptime.c
> > +++ b/fs/proc/uptime.c
> > @@ -4,13 +4,19 @@
> > #include <linux/sched.h>
> > #include <linux/seq_file.h>
> > #include <linux/time.h>
> > +#include <linux/kernel_stat.h>
> > #include <asm/cputime.h>
> >
> > static int uptime_proc_show(struct seq_file *m, void *v)
> > {
> > struct timespec uptime;
> > struct timespec idle;
> > - cputime_t idletime = cputime_add(init_task.utime, init_task.stime);
> > + int len, i;
> > + cputime_t idletime = 0;
>
> cputime_zero, I guess..

Yes, this needs to be cputime_zero.

> > + for_each_possible_cpu(i)
> > + idletime = cputime64_add(idletime, kstat_cpu(i).cpustat.idle);
> > + idletime = cputime64_to_clock_t(idletime);
> >
> > do_posix_clock_monotonic_gettime(&uptime);
> > monotonic_to_bootbased(&uptime);
>
> This is a world readable proc file, adding a for_each_possible_cpu() in
> there scares me a little (this wouldn't be the first and only such case
> though).
>
> Suppose you have lots of cpus, and all those cpus are dirtying those
> cachelines (who's updating idle time when they're idle?), then this loop
> can cause a massive cacheline bounce fest.
>
> Then think about userspace doing:
> while :; do cat /proc/uptime > /dev/null; done

Well, don't do stupid things like that. That falls into the same
category as programs calling gettimeofday all the time.

--
blue skies,
Martin.

"Reality continues to ruin my life." - Calvin.